In a hard or floppy disk drive system, at least one read/write head is moved across a data disk so as to be positioned over a selected one of the large number of substantially circular, concentric tracks in which data is recorded and/or reproduced. In one embodiment, a servo head is moved simultaneously across a dedicated servo disk for providing servo information to control the operation of the disk drive system. In another embodiment, embedded servo information in the data disk is read by the moving read/write head. The heads are mounted on a head actuator so as to position the read/write head at the desired track and the head actuator in turn is driven by an actuator motor.
One operation of the disk drive system is a track access operation or track seek operation in which the heads are moved across one or more tracks from an initial position to a desired track, so as to access the desired track for reading or writing data thereon. It is common for the data to be recorded such that specific portions or sectors of the data are recorded in specific ones of the tracks. Each such sector includes header data which in turn includes the respective track number, so that the read/write head, when accessing a track, can reproduce the track number, whereby the system knows at which track the read/write head is positioned. If a user wishes to access track 200 to read or write data, the system can move the read/write head until it is positioned over a track from which it reproduces the track number 200.
It is undesirable for the read/write head to have to read the track number in each track crossed during the track seek operation, as this would require a relatively long period of time during which the header data is read and the track number is extracted and compared with the desired track number. However, at the start of the track seek operation, the system knows that the read/write head is positioned at a particular track, for example at track 100, and that the servo head (if a dedicated servo disk is used) is positioned at a corresponding position on the servo disk. The system can move the read/write head from track 100 to track 200 by crossing 100 tracks, using the servo head to count tracks crossed from a servo pattern recorded on the servo disk. This significantly decreases the time required for the track seek operation. However, if the track count is inaccurate by reason of, for example, defects in the servo disk surface, the read/write head will end up positioned over the wrong track, for example track 198. It is then necessary for the system to perform at least a short track jump to track 200. Such a track jump adds significant time to the track seek operation. Therefore, it would be highly advantageous to provide a circuit for detecting the number of tracks crossed by the head during movement thereof across the disk which provides an accurate track count.